A first related art rebar structure includes columns and beams to be connected to the columns.
In a beam-column connecting portion of such a rebar structure where a column and beam are connected together (a panel zone), longitudinal bars (main reinforcement) and shear reinforcing bars for the column and longitudinal bars (main reinforcement) for the beam are arranged, and concrete is placed thereon. According to Standard for Structural Calculation of Reinforced Concrete Structures, the first impression of the eighth edition (Edited by Architectural Institute of Japan), generally, the beam-column connecting portion is designed in accordance with allowable shearing force Q which can be obtained from the following equation.
Q=κ(f−0.5) bD (κ: a coefficient according to the shape of the beam-column connecting portion, f: short-term allowable shearing unit stress of concrete, b: effective width of beam-column connecting portion, D: column depth)
For example, in a rebar structure disclosed in JP 3147699 U, column longitudinal bars include a normal strength portion having a given strength and a joint section and a high strength portion having a higher strength than the given strength. The normal strength portion is arranged in the central portion of the column longitudinal bars, and the high strength portion is arranged in the portion to be connected to a beam. In such column longitudinal bars, the ends of the normal strength portion are connected together by joining means such as welding.
However, in the generally-designed rebar structure, in order to increase the allowable shearing stress (shearing proof stress) of the beam-column connecting portion thereof, it is required to either increase the strength of the concrete by changing its base material or, as can be understood from the above equation, increase the section area of the beam-column connecting portion by increasing the column depth D.
When the concrete strength is increased, the cost of the structure is increased. Also, when the section area of the beam-column connecting portion is increased, the section areas of the entire column and the entire beam are increased, which narrows a living space.
According to JP 3147699 U, the longitudinal bars are partially reinforced to solve the problem of reliably connecting the reinforcing bars having different strengths, but JP 3147699 U does not address the narrowing of the living space.
A second related art rebar structure uses reinforcing bars in columns and beams, and in its beam-column connecting portion where a column and a beam are cross-connected together, column longitudinal bars arranged in the column and beam longitudinal bars arranged in the beam are connected together, and shear reinforcing bars are further arranged at this portion.
At this beam-column connecting portion, in addition to the axial force of the column acting thereon, and also forces generated due to repeated application of loads to the column and the beam act on the portions of the column corresponding to the upper and lower portions of the beam-column connecting portion, which may cause cracks in the concrete placed on the rebar structure thereby reducing the strength of the column. Especially, at the time of an earthquake, the displacements of the beams are larger than those of the columns and, when great force in the vertical direction (direction perpendicular to the columns) is applied to the columns due to the beam displacements, the portions of the columns corresponding to the upper and lower sides of the beams (that is, the portions of the columns corresponding to the upper and lower sides of the beam-column connecting portion) are caused to crack. In order to prevent such crack or the like, it is necessary to reinforce the beam-column connecting portion.
For example, in a rebar structure disclosed in JP 2010-236217 A, in order to prevent cracks in the upper and lower ends of the beam-column connecting portion from developing, reinforcing bands are provided to surround the column longitudinal bars.
However, in the rebar structure of JP 2010-236217 A, the reinforcing bands are provided on the upper and lower ends of the beam-column connecting portion, so that the reinforcing bands adjoin the beam longitudinal bars, and the working efficiency for providing the reinforcing bands is not taken into consideration.
Also, since the reinforcing bands are provided on the upper and lower ends of the beam-column connecting portion, the reinforcement of the columns with respect to the shearing stress applied to the columns is not always sufficient. Also, there is a demand for a reinforcement of larger columns.
A third related art rebar structure includes a plurality of longitudinal bars extending in an axial direction and a plurality of shear reinforcing bars surrounding the longitudinal bars for reinforcing the shear strength thereof. When there is a difference between the amount of the longitudinal bars in the end portion of the member and the amount of the longitudinal bars in the central portion of the member, the longitudinal bars may be arranged along the entire length of the member, in order to prevent the longitudinal bars from slipping and moving out of the inside of the reinforced concrete member even when the longitudinal bars receive bending tension. However, this increases the amount of the longitudinal bars, requires additional parts for connecting the longitudinal bars to each other, and increases the workload for connecting the longitudinal bars. Also, the amount of the shear reinforcing bars may be increased to improve the bond strength between the longitudinal bars and the concrete so as to suppress the slipping of the longitudinal bars. However, this increases the amount of the shear reinforcing bars and thus increases the workload for arranging the shear reinforcing bars.
In view of this, bond reinforcing bars may be used in addition to the shear reinforcing bars. For example, in a rebar structure disclosed in JP 4151245 B2, a longitudinal bar is surrounded by a bond reinforcing bar, or, a plurality of longitudinal bars arranged inwardly of longitudinal bars arranged at the outermost periphery of the structure are surrounded by a bond reinforcing bar.
However, in the rebar structure as disclosed in JP 4151245 B2, although a longitudinal bar is surrounded by a bond reinforcing bar, or, a plurality of longitudinal bars is surrounded by a bond reinforcing bar, a longitudinal bar arranged in the outermost periphery of the structure is not surrounded by the bond reinforcing bar. Therefore, there is a limit to providing sufficient reinforcement of the outermost peripheral side of the rebar structure that receives the shearing force the most.